On the other hand, Dirac has formulated the relativistic quantum mechanics (RQM) for a massive electron with spin 1/2 [1]}. In his unified theory of quantum mechanics and special relativity, a new particle solution has been proposed to allow the electron having
either positive or negative energy solutions. In particular, the positron corresponding to the negative energy solution has
been theoretically predicted and later has been experimentally confirmed [2]}. Note that the positron has a negative
charge opposite to the electron positive one.
